Mercapto acids are well-known derivatives which have a thiol (--SH) function and an acid (--COOH) function. They have many applications. Thioglycolic (or mercaptoacetic) acid, for example, may be used in its acid form as an intermediate in the synthesis of many pesticides and pharmaceutical products, or in its acid or salified (especially ammonium, amine, sodium, potassium or calcium salt) form for the pickling of metal surfaces, the treatment of sulphide ores and the treatment of leathers and hides; in the cosmetics industry, it also constitutes the most widely used reducing agent for the permanent-reshaping of hair (curling or straightening) and the main active substance in depilatory milks and creams. Similarly, thiolactic acid (2-mercaptopropionic acid) is used as a reducing agent for the permanent-reshaping of hair or as a constituent of depilatory milks and creams.
Pure mercapto acids have a slight pungent odor which is not really unpleasant. However, they always contain, in practice, sulphide compounds such as hydrogen sulphide and low molecular weight mercaptans, especially methanethiol or ethanethiol, which have an especially unpleasant nauseating odor. Very small amounts of these sulphide compounds are sufficient for their presence to be detected using one's sense of smell, the nose being, in this case, the best instrument of detection.
The presence of these malodorous compounds is associated with various processes of decomposition of the mercapto acids, which processes are still very poorly understood but are doubtless due to both ionic and free-radical mechanisms that can take place when air is absent. This decomposition and the resulting formation of malodorous compounds can, moreover, be monitored over time by various analytical techniques, especially by the so-called headspace method in gas chromatography.
In the various applications of mercapto acids, and more especially in their cosmetic applications, the odor emitted by the products employed constitutes a genuine nuisance to the users. An effort has hence been made to mask the odor of mercapto acids by perfumes, but this odor is, in general, too powerful to be amenable to satisfactory masking.
The proposal has also been made, in Japanese Patent Application No. 82-136,280 published under No. 84/027,866, to deodorize thioglycolic acid, pure or mixed with water, by extraction with a C.sub.4 -C.sub.8 non-aromatic hydrocarbon. However, it was found that, while this extraction process enables the malodorous compounds to be extracted and a deodorized acid to be obtained, the deodorization effect obtained is not lasting with the passage of time, since the malodorous compounds quickly re-form and cancel out the benefit obtained by the treatment; in some cases, the odor even returns at a higher level than the initial level.
Moreover, it is known to use supercritical fluids as extraction fluids. Supercritical fluids are phases which are at a higher temperature and pressure than critical values characteristic of the compound in question; in the supercritical region, there is no liquid/gas equilibrium curve separating a region in which the compound might be gaseous from a region in which it might be liquid. Extraction with a supercritical fluid affords many advantages; this is especially true when carbon dioxide is used; there is no contamination with a residual solvent; the critical temperature of CO.sub.2 is close to room temperature, enabling extraction to be performed under mild conditions and in a non-oxidizing environment; and CO.sub.2 is a non-inflammable and non-toxic liquid. Such extraction processes are, for example, described in P. HUBERT and O. G. VITZHUM Angew. Chem. Int. Ed. Engl 17, 710-715 (1978); F. TEMELLI et al, Food Techn. 42 (6) 145-150 (1988); D. A. MOYLER Supercritical fluids for extract preparation in Distilled Beverage Flavor--Recent developments, J. R. PIGOTT and A. PATERSON, Chichester Editions, England, Ellis HORWOOD Ltd (1989); J. W. KING, Cosmetic and Toiletries, 106 (8) 61-67 (1991). In all these processes, a gaseous extract containing the desired substance and a liquid extraction residue, which can be discarded, are obtained. Furthermore, as noted in the J. W. KING paper cited above, the treated products are non-polar compounds, similar to lipids and very hydrophobic.
The present invention relates to a process for deodorizing mercapto acids by extraction using a supercritical fluid; such a process enables a lastingly deodorized product to be obtained.